Methanol synthesis is a strongly exothermic and equilibrium-limited reaction. Increases in reaction temperature tend to disfavor methanol formation, and tend to deactivate some of the more commonly used copper based catalysts. Thus, controlling the reaction temperature and maintaining favorable equilibrium in the methanol reaction process is important.
In large-scale methanol plants, a gas phase synthesis reactor such as a cooled tubular reactor or a multistage adiabatic reactor is typically used. Because, the reaction to methanol is strongly exothermic, and efficient heat removal is a problem, this limits the range of composition of the feed gas that can be treated in a tubular reactor. For example, CO rich gases are very exothermic and difficult to process. Inefficient heat removal leads to hot zones in the reactor, and the catalyst may deactivate faster. Another problem is by-product formation being promoted at high temperatures. For example the production of ethanol and methyl formate increases at increasing temperatures, and alcohols, esters and ketones are difficult to separate from methanol-water mixtures being withdrawn from the process due to the formation of azeotropes.
Slurry bubble reactors are also used in the manufacture of alcohol. These beds involve bubbling synthesis gas feed into a reactor containing liquid in which methanol synthesis catalyst is maintained in suspension. Methanol is formed as the synthesis gas bubbles up through the liquid and contacts the catalyst.
U.S. Pat. No. 5,348,982 discloses a three phase slurry bubble column reactor. The three gas, liquid and solids phases are determined by the type of chemical reaction in the column. A gas is injected into the slurry bubble column at a velocity so that the solid phase is fluidized while maintaining plug flow over the column length.
U.S. Pat. No. 6,608,114 discloses making methanol using a slurry bubble reactor and dehydrating the methanol to make dimethyl ether. The reactor produces the methanol at relatively low water content to facilitate dimethyl ether formation.
U.S. Pat. No. 6,642,280 discloses a control scheme for conversion of variable composition synthesis gas to liquid fuels in a three-phase or slurry bubble reactor. The control scheme allows constant liquid product production and constant or limited purge gas emission using a variable synthesis gas feed condition. The control scheme provides for adjusting one or more of recycle ratio, water addition, and bypass flow.
U.S. Pat. No. 6,881,759 discloses a liquid phase process for the production of methanol being carried out in a slurry-bed reactor. The methanol product is used as a catalyst suspension liquid medium. The process is cooled using cooling tubes and produces medium or low pressure steam.
The use of slurry bubble columns for methanol or dimethyl ether synthesis typically requires removal of heat from the reactor to control the reaction temperature. Removal of heat from the reactor can be accomplished using a reactor that has sufficient heat transfer surface area and/or a heat transfer medium within the reactor. Tubes, coils, or jackets are examples of heat transfer surface area to transfer heat. Constructing a reactor that contains a slurry of catalyst, as well as provides heat transfer, is complex. What is needed is a process that provides a high level of control of low-temperature slurry bubble column reactors at a relatively low cost.